The present invention(s) relates to configurations for controlling inverters used to feed power to loads. The loads may include, inter alia, power grids and motors.
Inverters are used when delivering energy into power grids. For example, energy generated by solar cells or windmills can be transferred to a grid for distribution. The inverters convert the energy into sinusoidal currents useful for alternating current powered devices.
In other instances, inverters can be used to power, e.g., four quadrant motor drives such as may be used with permanent magnet (PM) motors.
Typically, the connection between a grid and the inverter consists of a three-phase pulse width modulated (PWM) inverter output feeding an inductor network, which smoothes the PWM voltage to produce a sinusoidal current on which is superimposed remnant PWM carrier frequency ripple current. This ripple current component can be shunted through a harmonic filter capacitor, resulting in a pure sinusoidal current which can be fed into the power grid. Inverter controls normally are used to regulate either the inverter or the grid current to control power flow and power factor.
Similar systems are used to control torque in PM motors and generators, at least where low acoustic noise is desirable.
FIG. 1 depicts a conventional inverter to power grid connection. The three phases of the power output by the inverter are respectively feed into smoothing inductors L1, L2 and L3. L4, L5 and L6 represent the inductance of the power grid.
Nodes N1, N2 and N3 represent the point of connection between the inverter smoothing inductors and the power grid.
Also respectively connected to the nodes N1, N2 N3 are harmonic filter capacitors C1, C2 and C3. The remaining inductors L7, L8 and L9; resistors R1, R2 and R3 and capacitors C4, C5 and C6 act as a damping network so that the harmonic filter does not introduce undamped second order resonance into the grid. As is known, these components can be replaced by active damping means.
FIG. 2 depicts currents A, B and C generated by a conventional inverter. FIG. 3 depicts typical line current A, B and C available in a power grid using a conventional inverter control configuration.
Traditionally, such inverters are controlled by employing a synchronous reference frame current regulator as the innermost control loop as shown in FIG. 4. In FIG. 4 there is illustrated a conventional inverter control schematic, with control loops shown.
In such a configuration, using well-known direct-quadrature-zero (dq0) transformation analysis, the fundamental component is transformed to a DC component where the Q axis current corresponds to the real component of current and the D axis current corresponds to reactive current. In the synchronous reference frame, therefore, a proportional-integral (PI) regulator will produce zero steady state error.
Typically the outer loops of such a converter generates a real component current reference Iqr. Such outer loops might include an inverter DC bus voltage regulator, or a motor speed regulator, or an external input commanding a defined current, torque, or power. Orientation of the synchronous reference frame is obtained using a phase locked loop (“PLL”) which regulates the D axis voltage to zero by establishing an orientation frequency and angle.
FIG. 4 depicts in a conventional control loop where a set point (“SP”) is compared against a feedback value. As FIG. 4 demonstrates, a current set point from an appropriate regulator, e.g., a speed, voltage, power or torque regulator, is compared against feedback variable Iqd obtained from the plant being controlled. The comparison yields variable Iqd.err which is fed into a current regulator to generate output voltage signal Vqd*. Voltage signal Vqd* is then fed into the inverter which in turn generates a voltage signal Vqd.
Voltage Vqd is fed into the inverter inductors as described above to generate current signal Iqd. Signal Iqd is filtered, as mentioned above, to generate voltage Vqd.flt used to power a grid or motor. However, the grid or motor is also subject to its own load voltages or back electromagnetic force (“emf”) which negatively affects the power available. As illustrated, this negative impact is subtracted from the voltage Vqd.flt and filtered again by the load inductance to finally produced the load current I.Load.
The performance of the control configuration can be measured by considering the resultant current signature. In that regard, the performance of the inner current loop control can be measured by considering the step response of the control, that is, how quickly the regulators can bring the actual inverter or grid current to follow an instantaneous change in current reference. The step response is best observed in the synchronous frame.
FIG. 5 is a graph representing the current and voltages fed back using a traditional control approach of FIG. 4 when the grid impedance is low. As FIG. 5 depicts, the inverter D axis current 502 is around 0, the inverter Q axis current 504 is between approximately 400 and 600 amps, the grid D axis voltage 506 is between approximately 0 as well, the grid Q axis voltage 508 is approximately 400 volts, the axis D line current 510 is between approximately 75 and 150, and the axis Q line current 514 is between approximately 400-600 amps which is substantially the same as the inverter Q axis current. As the figure depicts, a step change to the Q axis current 504 occurs at approximately 0.25 seconds which results in a well controlled, rapid step response to the new commanded value.
FIG. 6 is a graph representing the current and voltages fed back using the same control approach, with a grid or motor impedance (L4-L6 in FIG. 1) that is larger than the inverter output impedance (L1-3) in FIG. 1). As FIG. 6 depicts, the inverter D axis current 602 is around 0, the inverter Q axis current 604 is between approximately 400 and 600 amps, the grid D axis voltage 606 is between approximately −200 and −50, the grid Q axis voltage 608 is approximately 400 volts, the axis D line current 610 is between approximately 75 and 150, and the axis Q line current is between approximately 400-600 amps. As the figure depicts, a step change to the Q axis current 604 occurs at approximately 0.25 seconds which results in a controlled step response to the new commanded value.
As FIG. 6 shows, the step response is slower than it was in FIG. 5, and it is followed by a sustained oscillatory behavior. Also, in FIG. 6, a significant increase in the D axis current disturbance is shown, and there appears to be a limit cycle in the feedback even before the transient, associated with the increase in harmonic filter voltage disturbances. Additionally, it is no longer possible to use active damping to eliminate the passive damping components in the harmonic filter.
FIGS. 5 and 6 are results obtained by simulating the above described control systems using simulation software, namely VisSim™ available from Visual Solutions Inc, 487 Groton Road, Westford, Mass. 01886. In FIG. 5, the inverter inductance is 150 μH, while the line inductance is 100 μH. In FIG. 6 the line inductance is 600 μH. Accordingly, FIG. 5 depicts a situation when the inverter inductance is higher than the line inductance and FIG. 6 depicts a situation when the line inductance is larger than the inverter inductance. FIG. 6, in particular shows the sort of distorted grid currents that result from the lack of good current control when the line inductance is higher than the inverter inductance. Similar behavior can be observed in PM motor/generator applications with damped harmonic filters. Further, as FIG. 6 shows, the performance of the traditional control strategy is significantly compromised in situations when the inverter inductance is higher than the line inductance.
There are two reasons for the poor transient and steady state behavior shown in FIG. 6. The first is due to the inability of the controls to properly estimate the transformation angle used to transform currents and voltage to the synchronous DQ reference frame. This is typically accomplished with a phase locked loop (PLL)
A typical phase locked loop is shown in FIG. 7. This PLL is utilized in PM motor drives when an encoder is not present as well as in grid connected applications. When an encoder is present in the drives the PLL is not necessary. In one embodiment that is consistent with the present invention(s), the PLL transforms a three phase voltage signal into a synchronous reference frame having a Q axis voltage component VQE, and a D axis voltage component, VDE.
In typical applications the inverter inductance (L1-L3 in FIG. 1) is larger than the grid inductance (L4-L6 in FIG. 1) of the motor/generator inductance. In this case the rapid step responses shown in FIG. 5 are possible, and it is not difficult to provide active damping to enhance the stability of the filter or to allow removal of the damping components.
Typically the line voltage for orientation is sensed at the harmonic filter capacitors, nodes N1, N2, N3 in FIG. 1, and the actual grid voltage may not even be available (e.g., in the case of PM motors). Or, the line voltage may be sensed at a point of common coupling of the equipment to the grid (in which case the inductance L4-L6 would be representative of a transformer or other grid impedances upstream of the point of common coupling).
However, as power converter technology improves, and switching frequency increases, it is possible to reduce the inverter inductance so that it becomes significantly smaller than the grid or motor/generator inductance. This happens in PM motor applications where the inverter inductance typically might be less than 5% per unit (p.u.) while the motor/generator inductance might be as high as 20% per unit. Another instance where this can happen is in connection with a wind turbine where each turbine is fed by its own transformer. Transformer impedances are typically 5% to 10% per unit, while inverter output inductance may be lower than 2% per unit when the switching frequency is raised above 8-10 kHz.
Another reason for the poor performance shown in FIG. 6 is due to is the inverter current typically being measured and fed back to the current regulator, while, when the grid inductance is significantly higher than the inverter inductance, it is the grid current, which is not directly controlled, that has the slowest dynamic response.